19.1 The Spectrophotometer -6• MonochromatorConsists:• lenses or mirrors: focus the radiation• entrance and exit slits: restrict unwanted and control the spectral purity of radiation.• dispersing medium: separate the λ of polychromatic radiation from the source. (a) prism and (b) diffraction grating see Fig 19-2
19.1 The Spectrophotometer -71) Monochromator a. entrance slit b. collimating mirror or lens c. a prism or grating d. focal plane e. exit slit
19.1 The Spectrophotometer -9• Monochromator nλ = a – b (n = ±1 first-order….) Grating equation : nλ = d (sinθ – sinφ ) Filters: select a desired wavelength
19.1 The Spectrophotometer -10• MonochromatorChoosing the bandwidth: exit slit widthResolution trade-off Signal
19.1 The Spectrophotometer -101) Detector Convert radiant energy (photons) into an electrical signal Ideal detector : high sensitivity, high signal/noise, constant response for λs, and fast response time.
19.1 The Spectrophotometer -11 3) Detector Detector response depends on the λ of the incident photons.
19.1 The Spectrophotometer -12Photomultiplier tube: very sensitive detector
19.1 The Spectrophotometer -13Photodiode array spectrophotometer :records the entire spectrum at once. vs. Dispersive spectrophotometer: one λ at a time • speed (~1s/spetrum) • excellent λ repeatability • measure λs simultaneously • relatively insensitive to errors from stray light • relatively poor resolution (1~3 nm) vs 0.1nm
19.1 The Spectrophotometer -14 diode array spectrophotometer
19.2 Analysis of a mixture -1• Absorbance of a mixture : A = exb[X] + eyb[Y] + …
19.2 Analysis of a mixture -2• Isosbestic points : for rxn: X → Y, every spectrum recorded during chemical reaction will cross at the same point. Good evidence for only two principle species in rxn. Ex: HIn In- + H+
19.2 Analysis of a mixture -3Why isosbestic point? A 465 = ε 465 [ HIn ] HIn [ ] A 465 = ε 465 In − In − when [ HIn ] = [ In ] ⇒ ε − 465 HIn = ε 465 = ε 465 In − ∴ For a mixture : HIn In − [ ] A 465 = ε 465 b [ HIn ] + ε 465 b In − = ε 465 b ( [ In ] + [ HIn ] ) − −
19.4 What happens when a molecule absorbs light ? Four types of electronic transitions σ* π* E n 200~700 nm π 150~250 nm σ < 125 nm
19.4 What happens when a molecule absorbs light ? -5 1) Singlet / Triplet excited states ground excited excited singlet state singlet (S1) triplet (T1) E: T1 < S1
19.4 What happens when a molecule absorbs light ? -61) Electronic transition of formaldehyden → π* (T1), absorption of light at λ = 397 nm green-yellown → π* (S1), absorption of light at λ = 355 nm colorless (more probable)
19.4 What happens when absorbs light ?• Vibrational & Rotational states of CH3CO (IR and microwave radiation)
19.4 a molecule absorbs light 1) What happens to absorbed energy
19.4 a molecule absorbs light 7) Luminescence procedures : emission spectrum of M* provides information for qualitative or quantitative analysis. Photoluminescence : • Fluorescence : S1 → S0, no change in electron spin. (< 10-5 s) • Phosphorescence : T1 → S0, with a change in electron spin. (10-4~102 s) b. Chemiluminescence : Chemical reaction (not initiated by light) release energy in the form of light. ex : firefly.
19.4 a molecule absorbs light 1) In which your class really shines ? emission spectrum
19.4 a molecule absorbs light1) Absorption & Emission Spectra
19.5 Luminescence in analytical chemistry1) Instrument • .hνout (photon) • heat hνin • breaking a chemical bond
19.5 Luminescence• I = kPoC incident radiation sensitivity by P0 or C 6) more sensitive than Absorption
19.5 Luminescence4) Fluorimetric Assay of Selenium in Brazil Nuts – Se is a trace element essential to life: destruct ROOH (free radical) – Derivatized: – Self-absorption: quench